Process for protecting metals against corrosion at elevated temperatures



United States Patent 12 7 Claims. (a. 204-147 The present invention relates to the protection of metals against corrosion at high temperatures, more particularly, to a process of protecting metals against such corrosion wherein a potential difference is applied between the metal to be protected and a substantially continuous layer of corrosion product formed on the external surface of this metal.

It is well known that most of the metals or alloys are corroded when brought to elevated temperatures in the presence of various types of gases and that the resulting corrosion is a very serious problem in industrial operations.

In order to solve this problem, it has been proposed to make use of special metals or alloys which are only very slightly susceptible to corrosion such as the so-called stainless steels. However, these metals or alloys are very expensive and, furthermore, do not exhibit in actual fact a sufficient resistance to corrosion at elevated temperatures.

It is therefore an object of this invention to protect metallic materials against corrosion in a dry medium at high temperature.

It is another object of this invention to avoid the use, under corrosive conditions, of materials made of expensive corrosion-resistant metals or alloys.

It is a further object of this invention to substantially reduce the corrosion effect of gases at high temperature on metals or alloys normally susceptible to such a corrosion.

These and other objects as may be apparent from the following description and claims are achieved according to the present invention by applying a potential difference between a negative electrode consisting of metal or alloy itself and a positive electrode consisting of a solid material in contact with the outer surface of the substantially continuous solid layer of the corrosion product formed on the metal or alloy which has to be protected. The positive electrode is made of -a material having a higher conductivity than that of said corrosion product and is non-fusible and substantially non-corrodible at the high temperature of use.

In the following description the metal or alloy to be protected will be referred to as metal for sake of brevity.

The electric current between the electrodes must have a well-defined direction, which prohibits the use of an alternating current. Only a direct cunrent, a pulsed unidirectional current or a rectified current can thus be used for the purpose of this invention.

The mere flow of such a current in the suitable direction results in the reduction or even in the complete disappearance of the corrosion phenomena.

The optimal density of current depends on the conductivity of the corrosion product and will increase as the conductivity increases. Nevertheless, in most cases, it will be advantageous to use current densities of at least 0.01 ma./cm.

However, a substantially complete protection of the metal can only be obtained by use of a particular value or range of the current density. This value or range depends on the type of metal to be protected and must "ice be determined in each case by means of a relatively simple experiment. In numerous cases the so-determined current density is very low and, accordingly, it is generally not convenient to make use of a high current density which only results in a higher cost of the protection without any increase in efficiency as compared to lower current density.

However, it has been observed that the use of high current density at the beginning of the treatment may.

be of interest since it provides means for rapidly 0btaining a substantially complete protection of the metal or alloy. When such a result is attained, the current density may then be reduced to very low values without any new increase of the corrosion to be observed.

In the case of certain metals or alloys, and particularly of the so-called stainless steels of standard quality, the current may even be completely interrupted during relatively large periods without any new increase of the corro-v sion to be observed. Such a result is of considerable industrial interest since it provides means for manufacturing metal pieces pretreated so as to protect them against corrosion during a subsequent use in the presence of corrosive gases at high temperatures, whereas the application of the general process according to the invention with low current densities requires the concomitant supply of current during the period of use of the metal under corrosive conditions.

This surprising and unexpected result cannot be fully explained on the basis of the present knowledge in the art. However, a tentative explanation would be that the corrosion product formed in a first stage after the current is supplied has a particular structure which prevents a new penetration of the gas toward-s the metal or at least a normal development of the corrosion process.

It would, however, be apparent to those skilled in the art that since the current density is proportional to the applied potential difference, the latter must have a sufficient value in order to obtain the desired current density. In most of the practical cases these potential differences are of the order of a few volts. However, the potential difference will always be chosen lower than that at which the insulating layer constituted by the oxidation product may be burnt out.

The presence of a layer of corrosion product of the metal between the latter and the positive electrode is. an essential feature for the practice of this invention. This layer will be preferably chosen as thin as possible but, nevertheless, sufficient to avoid any direct contact between the positive electrode and the metal to be protected. The corrosion product consists of a derivative of the metal to be protected, stable at high temperatures and formed by attack of the latter by means of a corrosive material or compound. This attack will be advantageously carried out at a high temperature and, for instance, at that temperature at which said metal will be subjected during its intended use. The corrosion product may be of a mineral nature such as, for instance, a salt, a sulfide or an oxide of the metal. It may be either of the same nature as that which would be normally formed on the metal due to the action of the corrosive medium if said metal was not protected against corrosion according to the process of this invention, or of a different nature.

The corrosion product will be preferably selected to have the lowest possible permeability with respect to the corrosive materials, such as gases, so as to obtain the above mentioned permanent'protection.

The layer of the corrosion product, for instance, is easily obtained by contacting at high temperature the metal to be protected with a corrosive agent, preferably in the gaseous form and preferably in quiet atmosphere. A simple corrosion agent would be air.

A direct contact of the positive electrode with the corrosion product layer is strictly necessary for obtaining the protection of the metal against corrosion according to this invention since such a protection is not achieved satisfactorily when this condition is not fulfilled and as long as this electrode is entirely isolated from the corrosion product by a continuous coating of corrosive gas as thin as it may be.

Thus, the connection of the negative terminal of a current generator to the metal to be protected and of the positive terminal of said generator to an auxiliary electrode positioned in the corrosive gas in front of said metal, provides a negligible, if any, protection of the latter.

The positive electrode may be made of any material having a conductivity higher than that of the oxidation product and which is resistant to high temperatures. It

would be of advantage, however, to make use of an electrode having a high conductivity so as to reduce to a minimum the current losses by the Joule effect.

Thus, there may be used, for instance, an electrode of a non-corrodible metal, infusible at the temperature of use and particularly of a noble metal, which may have any shape including that of a grate, a spiral or a metal deposit even of the porous form.

According to another embodiment of this invention the positive electrode consists of a deposit of an oxide or another compound substantially more conductive than the corrosion product and not susceptible of being attacked itself by the corrosive gases.

It must be observed that in some cases a substantially complete protection of the metal is not immediately obtained as soon as the process of this invention is applied and even by use of high current densities. This is particularly the case when a wire or grate of non-corrodible metal is used as positive electrode. Such a phenomenon may be explained by an insufiicient contact between the positive electrode and the corrosion product at the beginning of the operation.

However, this period of relative inefiiciency of the process is very short and in most cases of the order of a few minutes or tens of minutes. An excellent protection of the metal is then rapidly obtained.

Nevertheless, when the positive electrode consists of a metal deposit on the corrosion product, the corrosion is stopped as soon as a sufliciently high current density passes therethrough. This tends to confirm that when using other types of electrodes the relative inefliciency of the process during the starting period is due to an insufficient contact with the electrode.

The present process may be advantageously used in numerous cases and where a metal or alloy is in contact with corrosive gases at high temperature, at which it is normally corrodible, for instance at temperatures higher than 500 C. and more generally higher than 800 C. As examplesof such gases there may be mentioned oxygen, air, carbon dioxide, steam, sulfur dioxide, hydrogen sulfide, the combustion gases as a general rule, the vapors of metalloids or acids, etc. These gases are either normally in the gaseous state or are produced in situ by decomposition or vaporization of solids or liquids such as, for instance, nitrates, chlorates, perchlorates, hydrogen peroxide or even liquid oxygen.

Thus such metals as iron, nickel, cobalt, chromium, molybdenum, tungsten, vanadium, zirconium, titanium, tantalum, aluminum, magnesium, manganese, copper and zinc either as such or admixed together or in the form of alloys with other elements including certain metalloids such as carbon or silicium may be efficiently protected by use of the process or treatment according to this invention.

Accordingly, the present process may be used for reducing the corrosion in furnaces, heat exchangers, nuclear reactors particularly those using carbon dioxide as cooling medium, gasor vapor-turbines, rockets, jets and turbo-jets and numerous other applications.

The following examples are given for illustrative purposes only to facilitate the comprehension of the invention and are by no Way to be considered as limiting in any manner the scope of this invention.

Example 1 A cylinder of tungsten is subjected to a preliminary oxidation by means of oxygen under atmospheric pressure at a temperature of 950 C. for 20 minutes so as to form on the external surface thereof an oxide layer about 0.4 mm. thick. The cylinder is then rapidly cooled and on its surface is wound a spiral of a platinum wire of a 0.1 mm. diameter, said spiral having a pitch of about 1 mm. By connecting the spiral to the positive pole and the cylinder to the negative pole of a direct current source while raising again the temperature of the cylinder to 950 C. in oxygen atmosphere, a flow of electric current through the oxide is observed.

The oxidation degree is expressed by the weight increase of the sample, which increase corresponds to the combination of oxygen with the metal. The weight in crease thus amounts to 11 rng/cm. during the 20 minutes of the preliminary oxidation phase. After 40 minutes from the moment of the current supply, with a density of 255 ma./cm. a further weight increase amounting to 18 Ing./om. is observed. During the 20 following hours the weight of the cylinder remains unchanged. The density of the current is then lowered to 30 ma./cm. and then to 0.05 ma./cm. and kept at said first value for 2 hours and said second value for 20 minutes without any change to be observed in the weight of the sample. The current supply is then interrupted and the weight of the sample starts increasing again at a rate of 10 mg./cm. in 20 minutes.

This experiment shows the efiiciency of the process according to this invention as well as the absence of effect when using a platinum wire alone without any current supply.

Example 2 Example 1 is repeated with a sample consisting of a cylinder of tungsten having a 2% thorium content and at a temperature of 1000 C.

After the 20 minutes of preliminary oxidation prior to the winding of the platinum wire on the cylinder, the corrosion degree amounts to 15 mg./cm. After the platinum wire has been laid the current density applied is chosen equal to 20 ma./cm. During the first day a further amount of corrosion of 2 rug/cm. is attained and thereafter the corrosion degree is nullified. At the end of the fourth day the current supply is interrupted and the corrosion starts again at a rate of 15 mg./om. in 3 hours.

A complete protection is also obtained with lower densities of current, down to 0.05 ma./cm. A still lower density of 0.025 ma./om. is, however, insufficient to completely protect the metal but it has bee-n observed that with this density of current the corrosion degree was reduced to 40% of its value in the absence of current.

Example 3 mg/cm. for the 6 following hours and is thereafter A complete protection of the metal may also be obtained with lower current densities of respectively 50 and 12 ma./cm.

When a complete protection of the steel is obtained by use of one of the above-mentioned current densities, the current supply may be interrupted and said complete protection is still maintained for several tens of hours.

With the current density of 5 ma./cm. the protection is not complete but the corrosion degree is reduced to one-tenth of its value in the absence of current.

Example 4 Example 1 is repeated with a sample consisting of a cylinder of brass having a 70% copper content and a 30% zinc content, and at a temperature of 850 C.

After 2 hours of a preliminary oxidation phase, the corrosion degree attains 4rng./cm. and after a density of current of 200 ma./cm. has been supplied it still amounts to 1 rug/cm. during the next twenty minutes. The corrosion degree is thereafter completely nullified. After 4 days the current supply is interrupted and the corrosion starts again at a rate of 4 mg./cm in two hours.

Example 5 Example 1 is repeated but with the use of an auxiliary electrode consisting of a very thin porous platinum deposit obtained \by coating the surface of the pre oxidized tungsten with a solution of platinum in aqua regia having added thereto hydroxylamine chlorhydrate, and heating the sample to a temperature of 500 C. up to the formation of a metallic platinum deposit of about 3 mg./cm.

After heating of the sample in air at a temperature of 1000 C., the corrosion degree in minutes, in the absence of current, attains 6 mg./cm. With a current density of 200 Ina/cm. the corrosion is immediate-1y stopped and no change in the weight of the sample has been observed in the following 54 hours during which the experiment was continued.

Example 6 With a current density of 12 rn a./cm. the weight increase of the bar is zero after 54 hours.

Example 7 The experiment according to Example 3 is repeated in a carbon dioxide atmosphere, under atmospheric pressure, with two identical cylindrical samples of the same stainless steel, each sample having a weight of 2.3 g., a height of mrn., and a diameter of 4 mm., said samples having both been preliminary subjected to the same preoxidation treatment by oxygen, and covered with a spiral of platinum wire.

One of these two samples is fed with current at a density of 50 ma./cm. and its weight remains unchanged after 18 hours, whereas the other, not fed with current has its weight increased by about 1 g. during the same period. An analysis of said last sample has shown that it was substantially entirely converted to oxide.

Example 8 Steel samples having a carbon content of 0.1% and a 17% chromium content have been electrolytically polished in a 10% perchloric acid solution in acetic acid.

After washing with water and drying in a nitrogen atmosphere, parts of the samples were brought to and maintained at a temperature of 950 C. in an oxygen atmosphere.

The weight increase of these samples was measured during this test. This increase amounted to 1.3 ing/cm. after 2 hours, 3.3 mg./cm. after 5 hours, and 14.3 mg./cm. after 65 hours. At the end of the, test no protection of the metal was achieved, the weight of the samples still continuing to steadily increase.

The other parts of the samples were first subjected to the same test for 5 hours. Their weight also increased by 3.3 mg./cm. Then the metal of the resulting samples was connected to the negative terminal of a generator, Whereas the positive terminal thereof was connected to a platinum wire spiral laid over the surface of the samples, on the preliminarily formed oxide layer. The samples were then brought again in an oxygen atmosphere to a temperature of 950 C., while causing a density of current of 1.5 amp/cm. .to pass Itherethrough. The weight of the spiral (unchanged throughout the experiment) being not taken into account, it has been observed that 60 hours after the beginning of the current supply (i.e. hours after the beginning of the test) the total weight increase of the samples amounted to 6.4 mg./crn. and remained unchanged at this level.

The current supply being interrupted at this moment, it was observed that after 91 hours from the beginning of the test the total weight increase of the sample was still unchanged (6.4 mg./cm. which shows that a complete protection of the metal has been achieved even after interruption of the current supply.

It will be understood that while there have been given herein certain specific examples of the practice of this invention, it is not intended thereby to have this invention limited to or circumscribed by the specific details of treated metals or alloys, corrosive media or operating conditions herein specified, in view of the fact that the invention may be modified according to individual preferences or conditions without necessarily departing from the spirit of this disclosure and the scope of the appended claims.

What is claimed as this invention is:

1. A process for protecting a metal at temperatures higher than 500 C. against the attack of a corrosive gas selected from the group consisting of oxygen, carbon dioxide, steam, sulfur dioxide and hydrogen sulfide, wherein the temperature of said corrosive gas and said metal is higher than 500 C., comprising the steps of:

(a) contacting said metal with said corrosive gas forming a substantially continuous solid dry corrosion product layer on said metal; and

(b) passing an electric current through said dry corrosion product layer from an anode positioned in direct contact with the external surface thereof towards the metal while maintaining the temperatures of the metal and the dry corrosion product layer higher than 500 C.

2. A process for protecting a metal at temperatures higher than 500 C. against the attack of a corrosive gas selected from the group consisting of oxygen, carbon dioxide, steam, sulfur dioxide and hydrogen sulfide, whereing the temperature of said corrosive gas and said metal is higher than 500 C., comprising the steps of:

(a) contacting said metal with said corrosive gas forming a substantially continuous solid dry corrosion product layer on said metal; and

(b) applying a potential difference between the metal acting as a negative electrode and a positive electrode in direct contact with the external surface of the dry corrosion product layer, said positive electrode being formed of a solid material having a conductivity higher than that of the corrosion product and the temperatures of the metal, the positive electrode and the dry corrosion product layer being maintained higher than 500 C.

3. A process for protecting a metallic surface at temperatures higher than 500 C. against the attack of a corrosive gas selected from the group consisting of oxygen, carbon dioxide, steam, sulfur dioxide and hydrogen sulfide, wherein the temperature of said gas and said metallic surface is higher than 500 C., comprising the steps of:

(a) contacting said metallic surface with said corrosive gas at a temperature higher than 500 C., forming a substantially continuous solid dry corrosion product layer on said surface; and

(b) passing an electric current of at least 0.01 men/cm. between said metallic surface acting as a negative electrode and a positive metal electrode in direct contact with the external surface of the corrosion product, said electric current being supplied by an external generator, and the temperatures of said metallic surface, said dry corrosion product layer and said positive metal electrode being maintained higher than 500 C.

4. A process according to claim 2 wherein said corrosion product is a member of the group consisting of the oxides, sulfides, and mineral salts of the metal to be protected.

5. A process according to claim 2 wherein said positive electrode is 'made of a material which, at the temperature of use, is infusible and substantially non-corrodible.

6. A process according to claim 2 wherein said positive electrode is made of a solid metal.

7. A process according to claim 1 wherein the current is supplied to the layer of the corrosion product at a density of at least 0.01 ma./cm.

References Cited by the Examiner UNITED STATES PATENTS 2,200,469 5/1940 Cox 204147 2,267,361 12/1941 Andrus 204-197 2,491,225 12/ 1949 Stearns 204-147 2,744,863 5/1956 Andrus 204-497 2,784,156 3/1957 Maurin 204l97 2,905,740 9/1959 Smyth et al. 136-83 3,001,919 9/1961 Petrocokino 204148 3,070,523 12/1962 Alexander et al. 204147 3,078,992 2/1953 Shapiro 204148 3,081,241 3/1963 Smith 212.7 3,102,086 8/1963 Cotton 204147 FOREIGN PATENTS 136,148 12/1959 Russia.

OTHER REFERENCES Evans, The Metal Industry, Dec. 21, 1928, pp. 589 and 590.

Herziog, Tran. of the Electrochemical Soc, 1933, vol. 64, pp. 87-98a.

Vernon, Tran. of the Electrochemical Soc., 1933, vol. 64, pp. 31-41.

JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, WINSTON A. DOUGLAS,

Examiners. T. TUNG, Assistant Examiner. 

1. A PROCESS FOR PROTECTING A METAL AT TEMPERATURES HIGHER THAN 500*C. AGAINST THE ATTACH OF A CORRESIVE GAS SELECTED FROM THE GROUP CONSISTING OF OXYGEN, CARBON DIOXIDE, STEAM, SULFUR DIOXIDE AND HYDROGEN SULFIDE, WHEREIN THE TEMPERATURE OF SAID CORROSIVE GAS AND SAID METAL IS HIGHER THAN 500*C., COMPRISING THE STEPS OF: (A) CONTACTING SAID METAL WITH SAID CORROSIVE GAS FORMING A SUBSTANTIALLY CONTINUOUS SOLID DRY CORROSION PRODUCT LAYER ON SAID METAL; AND (B) PASSING AN ELECTRIC CURRENT THROUGH AID DRY CORRESION PRODUCT LAYER FROM AN ANODE OSITIONED IN DIRECT CONTACT WITH THE EXTERNAL SURDFACE THEREOF TOWARDS THE METAL WHILE MAINTAINING THE TEMPERATURES OF THE METAL AND THE DRY CORROSION PRODUCT LAYER HIGHER THAN 500*C. 